Logunov DY, Dolzhikova IV, Zubkova OV, et al. - Researchers investigated the safety and immunogenicity of two formulations (frozen and lyophilised) of a heterologous COVID-19 vaccine which consists of two components—a recombinant adenovirus type 26 (rAd26) vector and a recombinant adenovirus type 5 (rAd5) vector; both components carried the gene for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike glycoprotein (rAd26-S and rAd5-S). They conducted two open, non-randomized phase 1/2 studies at two hospitals in Russia recruiting healthy adult volunteers (men and women) aged 18–60 years to both studies. In phase 1 of each study, either one dose of rAd26-S or one dose of rAd5-S was administered intramuscularly on day 0 and the safety of the two components was assessed for 28 days. Phase 2 of the study began no earlier than 5 days after phase 1 vaccination; a prime-boost vaccination, with rAd26-S given on day 0 and rAd5-S on day 21, was administered intramuscularly in the phase 2. From June 18 and August 3, 2020, 76 participants were enrolled to the two studies (38 in each study). Observations suggest a good safety profile of the heterologous rAd26 and rAd5 vector-based COVID-19 vaccine; the vaccine resulted in induction of strong humoral and cellular immune responses in participants. All participants produced antibodies to SARS-CoV-2 glycoprotein.